The present invention relates to the manufacture of continuous multifilament nylon yarns and more particularly relates to a high speed process for making fully-oriented nylon yarns and the resulting yarn products.
Continuous multifilament nylon textile yarns such as those of nylon 6,6 and nylon 6 are generally considered to be fully-oriented if they have elongations less than about 60%. While such yarns are used commercially for a variety of purposes, they are often used without texturing or bulking and thus are referred to as "flat yarns". Many are used in woven fabrics such as fabric for outerwear and also in warp knit fabrics such as fabrics for swimwear and auto upholstery. Dye uniformity in such fabrics is often critical to their value in use and it is generally desirable for fully-oriented yarns to be highly uniform to impart high dye uniformity to the fabric.
Known processes for making fully-drawn nylon yarns include the steps of extruding molten polymer, quenching the molten polymer to form filaments, coalescing the filaments to make a yarn and then drawing the yarn which reduces the elongation to the desired level. While the drawing can be done in a separate process, in most commercial processes used today the drawing step is integrated with the spinning step and such processes are called coupled "spin-draw" processes. Most conventional processes also include a relaxation step following drawing in which the tension on the yarn is reduced before winding-up, usually while heating the yarn.
One such known process for making fully-drawn yarn described in Swiss Patent No. 623 611. Swiss Patent No. 623 611 discloses the manufacture of nylon 6 yarns using a process in which the yarn is spun at 4000 meters per minute (mpm) (feed roll speed) and drawn in a draw step in which the unheated draw roll rotates at 5520 mpm. The yarn then undergoes a relaxation/entanglement step using a steam jet and wound is up at 4890 mpm.
If it is attempted to increase the speed of the process disclosed in Swiss Patent No. 623 611, the process has been found to be unsuitable for commercial use when the spinning speed (feed roll speed) substantially exceeds 4000 mpm. One problem which results at these speeds is a high number of broken filaments in the yarn. A second problem is yarn retraction on the package, i.e., the yarn retracts after winding with sufficiently strong forces to cause tube compression of, i.e., reduce the diameter or even crush an otherwise suitable tube core of cardboard construction. If the effect is severe enough, the resulting deformed yarn package with crushed tube core cannot be removed from the chuck on the wind-up without destroying the yarn.
One other problem with processes using unheated draw rolls as in Swiss Patent No. 623 611 is that the break elongation of the yarn generally cannot be reduced to less than about 50% without the number of filament breaks becoming unacceptable. Consequently, most yarn produced commercially using such processes has a break elongation of greater than about 50%.